Designs B/C

[bearasmith]

I'm confused. The rules say that the distance between the beams must be 5cm at any hight. Does that mean that bridges like this would be disqualified?

[chinesesushi]

Yes. It's a direct interpretation of the rules, and not an unofficial rules clarification. I believe it says that the bridge has to be at least 5 cm along the entire bridge, so that would be a violation.

[S4BB]

If the 2 lower beams are 5cm apart from each other for the entire span does that not qualify?

[bernard]

I see two possible interpretations for rule 4. e.: the bridge must be 5.0 cm wide for all heights (meaning the bridge I submitted to the Image Gallery would be have a construction violation) OR the bridge must be 5.0 cm wide along the whole bridge but this width can occur at any height. I will submit a FAQ for this.

[dholdgreve]

Seems to me the rules are very clear... at any cross sectional point along the 350 mm bridge span, there must be at least one part of the bridge that is at least 5 cm wide... it could be the top chord at one point, and the bottom chord at others... What concerns me is that a bridge could be constructed that is 3 CM wide, with very small 2 CM outriggers glued to one side or the other with a super light 1/64" member running from one to the other, serving no purpose other than to meet the width requirement.

[JonB]

Seems to me the rules are very clear... at any cross sectional point along the 350 mm bridge span, there must be at least one part of the bridge that is at least 5 cm wide... it could be the top chord at one point, and the bottom chord at others... What concerns me is that a bridge could be constructed that is 3 CM wide, with very small 2 CM outriggers glued to one side or the other with a super light 1/64" member running from one to the other, serving no purpose other than to meet the width requirement.

This would be a correct interpretation based on what was discussed at the coaches conference this summer. You will see bridges that are less than 5cm wide with "very small outriggers glued to one side or the other with a super light 1/64" member running from one to the other" to fulfill the 5cm requirement.

[chinesesushi]

Seems to me the rules are very clear... at any cross sectional point along the 350 mm bridge span, there must be at least one part of the bridge that is at least 5 cm wide... it could be the top chord at one point, and the bottom chord at others... What concerns me is that a bridge could be constructed that is 3 CM wide, with very small 2 CM outriggers glued to one side or the other with a super light 1/64" member running from one to the other, serving no purpose other than to meet the width requirement.

This would be a correct interpretation based on what was discussed at the coaches conference this summer. You will see bridges that are less than 5cm wide with "very small outriggers glued to one side or the other with a super light 1/64" member running from one to the other" to fulfill the 5cm requirement.

Well, the official rules weren't released in the summer. The old rules say "along the entire span." The new rules say "at any height along its span."

[JonB]

Seems to me the rules are very clear... at any cross sectional point along the 350 mm bridge span, there must be at least one part of the bridge that is at least 5 cm wide... it could be the top chord at one point, and the bottom chord at others... What concerns me is that a bridge could be constructed that is 3 CM wide, with very small 2 CM outriggers glued to one side or the other with a super light 1/64" member running from one to the other, serving no purpose other than to meet the width requirement.

This would be a correct interpretation based on what was discussed at the coaches conference this summer. You will see bridges that are less than 5cm wide with "very small outriggers glued to one side or the other with a super light 1/64" member running from one to the other" to fulfill the 5cm requirement.

Well, the official rules weren't released in the summer. The old rules say "along the entire span." The new rules say "at any height along its span."

This is true, but Mrs. Chalker was pretty adamant about this. It was asked and re-asked many times. I am in no way making an official clarification and I feel one will be made at some point. I am just going with how I interpret the rules and what Mrs. Chalker stated multiple times. Could the rules have changed since the summer? Sure, that is absolutely possible.

[nejanimb]

I had initially interpreted that rule to mean "at all heights, some part of the bridge must be at least 5cm wide." That is, if you held a ruler perpendicular to the span and moved it from the base up to the top of the bridge, some part of the bridge must protrude beyond a 5cm interval for the entire time.

But, come to think of it, it does seem like the usage of the word "any" is ambiguous. It may mean "any and all" (like "the supervisor checking spec can choose any") or "any such that one is sufficient" (like "the competitor submitting for check can choose any"). I really don't know here. Hm.

I'm curious if anyone here might be able to give insight as to why this parameter is included in the first place. I remember this rule was present for bridges when I did it the first go 'round, and it doesn't seem to make a whole lot of sense. Seems like an unnecessary restriction on design that doesn't increase the design challenge meaningfully, but just creates another annoyance in spec-checking.

(If my first interpretation is correct, I'd almost certainly *suggest* using a system that adds a useless extra piece just to fit this requirement! Which seems odd.)

[dholdgreve]

I think we may actually be over-simplifying it... Going to a more narrow design with a UOR (Useless Out-Rigger) seems to dictate that the loading block be placed on top the bridge. I believe this may force the design to include many more diagonal braces in the Z plane to stabilize it, as opposed to placing the loading block within the bridge supported off the bottom chord. This design virtually forces the bridge to exceed the 5 CM requirement, and may utilize the solid edge planes of the loading block as stabilizing diaphragms in lieu of the Z axis braces... just a thought... I still see no ambiguity in the way the rules are written... At some point, at some elevation along the entire span of the bridge it must measure 5 cm wide. Not the entire bridge... just one horizontal dimension of the cross section of the bridge. High, low, or in between... That part seems very simple.